The present invention relates to a coil system for use in the probe of a nuclear magnetic resonance spectrometer and, more particularly, to a coil structure that provides improved sensitivity in making measurements.
In an NMR spectrometer, a probe holes a sample in place in a uniform static magnetic field. A coil is disposed close to the sample within the probe to apply an exciting RF magnetic field to the sample. The resonance signal originating from the sample is picked up by the coil, and then it is fed to a receiver circuit. The output from the receiver circuit is furnished to a computer, which takes the Fourier transform of the signal to obtain an NMR spectrum. At this time, the sensitivity of the NMR spectrometer depends to a large measure on the degree of coupling between the coil and the resonators of the sample. Therefore, great care is taken in designing the shape and structure of the coil. An NMR spectrometer employing a superconducting magnet to create the static magnetic field has typically used as its probe coil a cylindrical saddle coil as shown in FIG. 1, where two spirally wound coil portions 1 and 2 are symmetrically arranged about the Z-axiz of a cylinder around a cylindrical volume of radius R in which a sample tube holding a sample is disposed. The coil portion 1 consists of straight portions 1A extending parallel to the Z-axis and arc-shaped portions 1B lying in planes that are perpendicular to the Z-axis. Similarly, the coil portion 2 consists of straight portions 2A and arc-shaped portions 2B. FIG. 2 is a cross-sectional view of the straight portions 1A and 2A taken on a plane perpendicular to the Z-axis. It can be seen from this figure that the straight portions 1A and 2A are disposed on the periphery of the circle of the radius R in a symmetrical relation with respect to Y-plane. These coil portions are used to set up an RF magnetic field in the direction of the X-axis.
The concept that underlies the prior art coil system as mentioned above is that two spiral coil portions 1 and 2 of the same shape are disposed on opposite sides of the cylindrical volume in a symmetrical relation with respect to the Y-plane. Consequently, the number of turns of such a coil system is 2, 4, 6, 8 or other even number. The prior art technique described thus far is disclosed in U.S. Pat. No. 4,398,149.
In recent years, static magnetic fields of larger strengths have been obtained by the use of superconducting magnets. As a result, the frequency observed has been raised from about 400 MHz to 500 MHz and 600MHz. In order to raise the frequency in this way, the sample probe coil is required to have a small inductance and a high Q and have the ability to raise the tuning frequency and enhance the signal-to-noise ratio and the sensitivity. Heretofore, a coil system having the least number, i.e., two, of turns as shown in FIG. 1 and showing the minimum value of inductance has been used to satisfy such requirements. However, even this coil system is unable to offer sufficiently low inductance. The present situation is that this coil system is employed with unsatisfactory result.
In view of the foregoing, the present inventors have already proposed a coil system of single turn as shown in FIGS. 3(a) and 3(b), where windings are arranged on the Y-axis of FIG. 2 in a manner quite different from the above-described prior art theory. (See U.S. patent application Ser. No. 714,580, filed on Mar 21, 1985.) FIG. 3(a) shows the shape after a conductive sheet has been blanked. FIG. 3(b) shows the condition in which the blanked sheet has been shaped into a cylindricl form. The coil system shown in FIGS. 3(a) and 3(b) comprise straight conductor portions 10 and 11 extending parallel to the Z-axis of the cylinder, first arc-shaped conductor portions 12 and 13 that connect together the straight portions 10 and 11 in series to form an annular conductor that constitutes a coil, second arc-shaped conductor portions 14, 15, 16, 17 which are similar in shape to the first arc-shaped portions 12, 13, and leads 18, 19 for connecting the coil to an external circuit. The second arc-shaped portions 14 and 15 are connected to the straight portion 10, while the second arc-shaped portions 16 and 17 are connected to the straight portion 11. The arc-shaped portions 15 and 16 are not connected together. Also, the arc-shaped portions 14 and 17 are not connected together. Therefore, these second arc-shaped portions 14-17 do not act as components of the coil. The first arc-shaped portions and the second, similarly shaped second portions are disposed symmetrically with respect to the center O of the cylindrical volume in which a sample is placed. Thus, the second arc-shaped portions are disposed to compensate for inhomogeneities of the static magnetic field.
The coil system shown in FIGS. 3(a) and 3(b) has only one turn and exhibits an inductance smaller than the inductance of the conventional coil system of two turns. In the single-turn coil, the straight conductor portions 10 and 11 that are components of the coil are disposed in the YZ-plane perpendicular to the axis of the RF field, or the X-axis. Since the straight portions 10 and 11 placed in this way feels the maximum value of strength of the RF magnetic field produced along the X-axis about the center O of the cylindrical volume, the coil should yield high Q. Consequently, the proposed coil system should exhibit small inductance and high Q. In practice, however, an actual tuning circuit incorporating such a coil system showed a Q lower than anticipated, for reasons explained below. In an actual NMR probe, the proposed coil system is placed at the center of the static magnetic field. Tuning capacitors are connected to leads which are brought out from this coil system to form a resonant circuit. That is, the capacitors are not directly attached to the coil system. Inductances of the leads and stray capacitance hindered the anticipated improvement in q of the tuning circuit, or the resonant circuit.